How do I get Formula of calculate quantile spec limits in process capability

[#1]

I do distributions analysis in jmp pro.18 with fit smooth curve function, then get USL/LSL value by using 6-sigma to calculate quantile spec limits under process capability function. I'd like to know what formula is used to calculate USL/LSL. I'm trying to run the same formula in the excel version. 

[#2]

Hi @wilsonliao233 

Check below link if it can answer your question.

Statistical Details for Capability Indices for Nonnormal Distributions

Find new spec limits by adjusting the CpK value - JMP User Community

The 6Sigma is equivalent to 1.413